This invention pertains to the passive detection of radiated noise from water-borne vessels. More specifically, it relates to detecting the presence of water-borne vessels by means of counting the number of upward crossings of the radiated noise signal received per unit time that are above a preselected threshold level, and comparing said number of counts with the expected upward crossing rate of background noise or some other noise reference. Prior art passive detection devices such as sonar detection apparatus are not capable of distinguishing between and identifying the variety of noise signals being received. Prior art sonar detection devices are only capable of receiving and detecting coherent signals in a noise environment.